The present invention relates to ice protection networks of heater elements used in anti-ice and de-ice systems for prevention and removal of ice from aircraft airfoils, and more particularly, for the balancing of electrical loads between the heater elements comprising the overall deicing system to accommodate malfunctions and selective heater element activation.
One of the problems encountered in modern day aviation is that of wing icing, where ice forms over the leading edges of the wings and control surfaces of an aircraft. Several methods have been devised to deal with this problem. One way is through the use of deicer boots installed along the leading edge of the airfoil. Deicer boots are typically constructed of neoprene and operate by causing it to change profile and break off the ice. Some use rotary devices that rotate within and against the internal surface of the deicer boot and some operate by expansion and relaxation of the deicer boot through use of compressed air. Either method will cause the deicer boot to expand beyond the standard airfoil profile and thus mechanically break the ice away from the leading edge of the airfoil. However, in order to break ice by using this device and method, it is first necessary to allow the ice to build up, or accrete, to the point where it will break away from the surface of the deicer boot without adhering thereto. Also, under extreme conditions, the ice may build up so fast around the boot between cycles that a cavity may be formed, in which the inner surface of the built-up ice in the cavity is just beyond the greatest extent of the deicer boot travel. In such conditions, ice may continue to build up while the deicer boot is rendered ineffective, since it only expands within the ice cavity and therefore has no effect on the built up ice.
Other de-icing methods apply engine bleed air directly onto the airfoil. High temperature and high pressure air is bled from the compression stages of an engine through pressure regulator valves and ducted through perforations on manifolds behind the leading edge inside the wing. The heat from this bleed air is sufficient to warm the wing to the point that it melts the ice interface, which causes the ice build-up to slide off the wing. However, a major disadvantage of this approach is that it reduces the efficiency of the engine, uses more fuel, and requires more complex designs to provide effective ducting.
Other anti-ice and de-icing designs in the prior art operate through electrical means. According to such designs, a heating blanket 115 is wrapped along the leading edge of an airfoil, typically the wing as depicted in FIG. 1, in order to provide resistive heating that melts ice as it forms along the airfoil. The heating blanket 115 may extend longitudinally along the leading edge 110 of the wing from the fuselage to the wing tip and horizontally along the upper and lower wing surfaces 130, 140 from the leading edge 110 a short distance towards the trailing edge 120 of the wing. The heater elements comprising the heating blanket 115 may be longitudinally divided into a plurality of sections from the innermost end of the heating blanket 115 in the proximity of the fuselage to the outermost end of the heating blanket 115 towards the wing tip. Each section is comprised of heater elements divided horizontally into zones (e.g. upper zone 150a-c, leading zone 160a-c, and lower zone 170a-c), so that each section will contain three separate zones. For example, zones 150b, 160b, and 170b comprise a section. This division scheme forms a network of heater elements called a wing ice prevention (WIP) heating network.
The heater elements may, in some cases, be formed as individual, electrically heated mats, or heater elements, adhered to the aircraft surface by an adhesive. U.S. Pat. No. 5,765,779, issued to Hancock et al., discloses a mat section that is configured in groups to a structural member of an aircraft, such as an airfoil. Each mat is resistively heated by an electrical power source and may be operated independently of other such mats. The disclosure states that the mats may be grouped for connection to respective phases of a multi-phase alternating current (AC) power supply. The disclosure shows a mat adhered to the surface of a wing.
Each heater element may also be provided with an ice detection mechanism for determining the amount and extent of ice that may be forming or formed in the section and zone that the heater element services. The prior art is replete with various devices used for the detection of ice. One such example is given by U.S. Pat. No. 4,514,619, issued to Kugelman, which discloses an electrical device for monitoring the current flow through the resistive elements contained in the mat comprising the heater element, so that individual mats may be activated or deactivated, i.e. turned on or turned off, whenever the circuit detects conditions conducive to ice formation or error conditions.
Each heater element may be configured to have the different heating capacity as determined by the location of the portion of the heating blanket 115, i.e. the heater element, along the wing and the collection efficiency at that location. The heat generated by the resistive heater element may prevent or remove ice formation in these sections.
The resistive heater elements are generally powered by an electrical power control unit (EPCU) of the aircraft, which supplies 3-phase power to the heating network. The EPCU may distribute single-phase power to individual heater elements as required for ice-protection operations.
Error or failure conditions present special problems for WIP networks. During standard operation, an individual heater element may fail through damage from foreign objects impacting the wing, material fatigue, or other causes. When one heater element fails, then all heaters in the section containing the failed heater element must be turned off and the corresponding section in the other wing must also be turned off for aerodynamic considerations. For example, if the upper heater element in a section fails through an open-circuit, then all power to heater elements in that section on both wings will be removed from the upper, leading edge, and lower zones. To maintain reasonable power quality specifications under failure conditions, it is desirable to limit phase imbalance to a maximum threshold value. In case of failure of the ice detection mechanism, which may require a manual activation of the WIP, it is desirable to have the ability to activate the lower zone heater elements first to remove the accreted ice on the lower surface. This may allow a portion of the ice to be removed from the lower surface before activating the remaining zones to remove ice from the leading edge and upper surface, and thus reduce the probability of large ice chunks from damaging the aircraft as they break away from the airfoil. Thus it is desirable to maintain a reasonable balance in load between the three phases of the power source while at the same time providing the capability to activate the lower zone separately when a malfunction occurs in the ice detection circuitry. The network shown in FIGS. 2 and 3 and the network shown in FIGS. 4 and 5 show two network arrangements currently in use to resolve these problems. The sections have been numbered from 1 to 10 for illustrative purposes.
For the WIP heating network shown in FIGS. 2 and 3, Phase A is configured to supply electrical power to 3 sections (i.e. sections 3, 9, and 10, containing a total of 9 elements), Phase B is configured to supply power to 4 sections (i.e. sections 2, 7, and 8 containing a total of 9 elements), and Phase C is configured to supply power to 4 sections (i.e. sections 1, 4, 5, and 6, containing a total of 12 elements.) Although the number of heater elements is different for each phase, the phase balance between Phases A, B, and C is maintained by having substantially equal total resistance in the heater elements in each of the three phases. For the WIP heating network shown in FIGS. 4 and 5, the power distribution is configured so that Phase A supplies power to upper zone, Phase B supplies power to the leading edge zone, and Phase C supplies power to the lower zone. In normal operation all three phases are balanced equally with 10 heater elements each.
If there is an open circuit in a heater element and it fails, then all heater elements in the corresponding sections on both wings must be turned off for aerodynamic considerations. Referring to FIGS. 2 and 3, the WIP heating network shown will experience a phase imbalance when all three heater elements in the section containing the failed heater element must be deactivated, since each section is powered by a single phase, which would have a high probability of exceeding the maximum allowable threshold for phase imbalance. However, the WIP Heating network shown in FIGS. 4 and 5 may remain phase balanced when all heater elements in a section containing a failed heater element must be shut down, since each section is provided power by all three phases. A maximum threshold may be established such that the difference between power drawn in any two Phases should be less than about 3.3% of the total load under normal operation for loads above 30 kVA, according to MIL-STD-704F.
Note that the WIP Heating network in FIGS. 2 and 3 allows for the switching off of separate zones across all sections in the event of an ice detection failure since each zone is phase balanced. So this network would remain phase balanced for the activation of a single zone. However, the WIP Heating Network of FIGS. 4 and 5 shows that all power to the upper zone is supplied by Phase A, all power to the leading edge zone is supplied by Phase B, and all power to the lower zone is supplied by Phase C. If only a single zone were activated, e.g. the lower zone powered by Phase C, then a severe load imbalance would result, which would in all probability exceed the maximum threshold value. It is desirable to establish a maximum threshold value such that the difference between power drawn in any two Phases should be less than about 1/N of the total load, where N is the number of sections in the WIP. Thus, for the case where there are 10 sections, this means that a maximum threshold value should be about 10%.
Hence, it can be seen that there is a need for a system and method of distributing three phase AC power to the elements of a WIP heating network in such a way that the load on each of the phases is balanced according to the acceptable tolerance value in the event of a heater element failure, yet the load on each of the phases remains balanced if only a single zone must switched on or off as a unit in the event of an ice detection circuit failure.